The present invention relates generally to a fixturing or workpiece holding and clamping device and method, and in particular, to a fixturing or workpiece holding and clamping device utilizing a viscosity increase or solidification of a magnetorheological fluid work contacting medium as a method to secure both regular and irregular shaped workpieces for precision machining or measuring operations.
The securing of irregularly shaped workpieces, such as jet engine or turbine blades having an enlarged end projecting from an elongated and considerably thinner airfoil section, for machining operations such that it does not result in damage to the workpiece has been found to be difficult. Typical methods of clamping through clamps or fixtures are not practical since they can cause permanent damage to the workpiece. As a result, the traditional solution to prevent damage to these and other irregularly shaped workpieces include encapsulation by the casting of a low melting point molten matrix material such as lead or zinc around the thin irregularly shaped portion of the workpiece, such as the airfoil section, after which the machining or measuring of the enlarged end portion is performed, as is seen in U.S. Pat. No. 5,947,662 to Becker et al. for "System For Holding Thin-walled Workpiece During Machining." Generally, this procedure involves inserting the elongated thin portion of the workpiece into a cast iron block having a cavity which is significantly larger than the workpiece itself. The molten matrix material is then poured into the cavity, surrounding and encapsulating the workpiece. After the matrix material cools and solidifies, the workpiece is secured in a fixed position for the machining or measuring operations. Upon completion of the machining or measuring operation, the matrix material is melted away from the thin irregularly shaped portion of the workpiece, leaving a finished product. This procedure, however, adds considerably to the expense of producing such workpieces, increases health and environmental risks associated with the vapors released from the molten matrix material, and fails to adequately protect the workpiece against deformation damage during the machining operations. Furthermore, such solutions cannot be applied to workpieces which are vulnerable to damage from the heating and cooling cycles associated with the addition and removal of the matrix material, or which have finished or treated surfaces which may become contaminated by residue from the molten matrix material.
Other solutions to the problem of securing irregularly shaped workpieces include the use of complex single-purpose hydraulic clamping devices such as is shown in U.S. Pat. No. 4,033,569 to Dunn for "Deformation-Preventing Workpiece-Holding Fixture for Machine Tools." These devices are typically suitable for holding only a limited range of irregularly shaped objects, and operate by applying a plurality of clamping members to a number of locations on the surface of the workpiece. Application of a clamping force to only a limited number of locations along the surface of a workpiece while retaining it during the machining operations can result in the buildup of stress or damage in the workpiece from the non-uniform application of the clamping forces.
A similar solution is exemplified by U.S. Pat. No. 3,818,646 to Peterson for "Fixture For Holding Precisely Shaped Parts" wherein an irregularly shaped workpieces, such as the thin elongated airfoil portion of a jet engine turbine blade, is secured for machining operations by a plurality of individual movable pins extending from the side wall of a clamping fixture to engage both the convex and concave surfaces of the workpiece. While increasing the number of individual movable pins extending from the side wall of the clamping fixture results in a more uniform application of clamping force to the irregularly shaped workpiece, this solution still fails to provide a completely uniform application of clamping force, and is limited to operation on workpieces having exterior surfaces with generally smooth curvature.
An alternative solution which applies a more uniform clamping pressure to the surfaces of a regular or irregular workpiece involves the use of a dry particulate material fluidized by a pressure of gas injection means for insertion of a workpiece, which is then substantially solidified by the application of a vacuum force or magnetic field to the dry particulate material. Examples of these types of fixturing devices may be found in U.S. Pat. No. 3,953,013 to Griffith et aL for "Method and Apparatus for Clamping A Workpiece In A Quasi-liquid Medium" and U.S. Pat. No. 3,660,949 to Cose, Jr., for "Work Holder For Irregular Shaped Workpieces." However, the use of a dry particulate fluidizable material or quasi-liquid requires a complicated variety of associated gas injection and vacuum generating elements, as well as containment for the dry particulate fluidizable material, since an excess of fluidizing pressure can easily expel the particulate material from the device.
A second alternative solution for applying a more uniform clamping pressure to the surfaces of a regular or irregular workpiece involves the use of electrofluids which respond to the presence of either alternating electric fields or a voltage difference by manifesting an apparent change in bulk viscosity. It is known that if these fluids are applied as a film over a dielectric surface, and an alternating electric field is applied to the fluid from beneath the surface, a workpiece placed on or in the electrofluid film causes the electrofluid to be energized by the electric field to secure the workpiece firmly in place. These devices, exemplified by U.S. Pat. No. 3,197,682 to Klass et al. require the application of a high voltage and potentially dangerous, three-phase current to the device, and do not permit workpieces to be immersed in the electrofluid film to any great depth, thereby limiting the clamping pressure of the device. Furthermore, electrorheological fluids are temperature sensitive, and typically have an inability to withstand water contamination, rendering them useless in machining applications wherein a machining tool is cooled by the application of water or other water-based liquid coolant to an exposed cutting surface.
Accordingly, there is a need in the industry for a self-contained fixturing or workpiece holding and clamping apparatus or device and method capable of securing both regular and irregularly shaped workpieces, such as jet engine turbine blades, for machining operations with a uniform clamping force so as to reduce the stresses associated with the machining operations on the workpiece, while also being easy to use, simple to construct, and which also eliminates the risk of environmental and workpiece contamination, as well as the risk to an operator's health from electric shock or the inhalation of harmful vapors or particles.
It is believed that an apparatus and method for immobilizing and securing both regular and irregularly shaped workpieces through the solidification or viscosity increase of a magnetorheological fluid subjected to a magnetic field will solve many of the problems associated with traditional work holding fixtures. It is known that in the presence of an appropriate magnetic field, solid magnetizable particles in fluids such as mineral oil, silicone oil, or other suitable organic liquid move into alignment, forming fibrous structures parallel to the applied field, significantly increasing the viscosity of the fluids and substantially decreasing the ability of the fluid to flow or be sheared.
A magnetizable carrier fluid or ferrofluid may be substituted for the mineral oil, silicone oil, or other fluid used as a carrier for the solid magnetizable particles in traditional magnetorheological fluids. While ferrofluids themselves do not solidify when subjected to an applied magnetic field, they similarly exhibit magnetic field-induced viscosity increases, and may be utilized to achieve yield stress levels significantly in excess of traditional magnetorheological fluids, as is taught by U.S. Pat. No. 5,549,837 to Ginder et al. for "Magnetic Fluid-Based Magnetorheological Fluids."
The basis for the magnetorheological effect can be explained by the inter-particle forces induced by the applied magnetic field. When an external magnetic field is applied to an initially random arrangement of magnetizable particles, a magnetic moment which is approximately parallel to the applied field is induced in each particle. The force between two particles whose moments are aligned head-to-tail is attractive, promoting the formation of chains or more complicated networks of nearly contacting particles aligned along the direction of the field, significantly increasing the viscosity and essentially solidifying the fluid. The strength of this solidified magnetorheological fluid can be characterized by the yield sheer stress at which the network of aligned particles is disrupted and the particles flow. Fluids having a high yield stress can sustain larger mechanical forces when solidified in the presence of a magnetic field before flowing. Magnetorheological fluids easily obtain yield stress values in excess of 5 psi in the presence of a magnetic field, and may be prepared to achieve yield stresses on the order of 20 psi as taught by U.S. Pat. No. 5,667,715 to Foister for "Magnetorheological Fluids." In general, for a magnetorheological fluid, it is known that an increase in the flux density of the magnetic field to which it is subjected will result in an increase in the yield stress, i.e. an increase in viscosity which in this context is understood to mean solidification.